various: add golangci-lint, fix issues (#7905)

This adds an initial and intentionally minimal configuration for
golang-ci, fixes the issues reported, and adds a GitHub Action to check
new pull requests against this linter configuration.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I8f38fbc315836a19a094d0d3e986758b9313f163

ipn/ipnlocal/network-lock_test.go

@@ -875,7 +875,7 @@ func TestTKAForceDisable(t *testing.T) {
 	}
 
 	if b.tka != nil {
-		t.Fatal("tka was re-initalized")
+		t.Fatal("tka was re-initialized")
 	}
 }